The Earth is our life support system, evolving over billions of years, presently providing life forms with the atmosphere and resources suitable for survival. In the 1960s James Lovelock considered that the Earth was like a living organism, itself engaged in a self-regulation to support its own "survival". His Gaia hypothesis, as it has become known, was defined by Lovelock according to the following idea.

"The chemical and physical condition of the surface of the Earth, of the atmosphere, and of the oceans has been made fit and is actively made fit and comfortable by the presence of life itself."

The idea of Gaia was formed after mankind entered space and began to remotely observe and understand features of planets using satellite technology. By observing Earth, Venus and Mars, Lovelock proposed that for life to exist, a planet�s atmosphere must have a very different composition to its surface, as with Earth. This assumption was based upon the idea that a given planet would, without life, be in a state of equilibrium and its atmosphere and surface would be very similar. Based upon this assumption, Lovelock suggested that life must be required to maintain the state of imbalance between the atmosphere and surface.

Lovelock went on to suggest that this imbalance is maintained by an active control system. This active control system quite simply reacts to changes within the Earth�s atmosphere, correcting these changes and allowing life to continue. This complex system of planetary control became known as Gaia, after the Greek Earth goddess. The active control system was life itself.

Gaia does not respond to environmental stresses in a simple way. Changes in the amount of solar energy received by the Earth as its orbit varies over tens of thousands of years, for example, have produced complex climatic fluctuations between cold Ice Age episodes and warmer interglacial periods during the last 2 million years. Over the much longer term, the gradual warming up of the Sun since the birth of the solar system 4� billion years ago has, Lovelock believes, placed an increasing amount of stress on Gaia. Consequently, as Gaia strains increasingly harder to maintain optimum conditions suitable for life, short-term stresses could yield abrupt and significant responses.

Although neat and simple, there is little conclusive proof of the presence of an active planetary control system on Earth. Unfortunately, geological evidence stretching back through Earth history is too patchy to provide any reliable confirmation of the Gaia hypothesis. Nevertheless, Gaia does illustrate how the Earth may respond to stresses placed upon it. Until recently, Earth has had to adapt to natural changes. Now, it may also have to respond to a significant impact from humans, through air pollution and a man-made interference with the global climate.